As above, conventionally to prevent heat-resistant stress fatigue crack (heat-crack) of a die (steel product) as much as possible, to treat the die by an SP treatment and give the die a compressive residual stress is well known (e.g., see Patent Documents 1 and 2).
Patent Document 1 discloses carrying out a plurality of shot-peening treatments of the die (a steel product) that was heat-treated, by injecting spherical particles (shots) having different hardness and different diameters, by suitably changing the hardness or the diameters of the particles (claim 3, etc.).
Patent Document 2 discloses using amorphous particles to be injected (shots) having a great hardness and a low Young's modulus in the shot-peening treatment of Patent document 1 (claim 3, etc.).
However, the dies in Patent Documents 1 and 2 are those used in forging or pressing (Paragraph 0001 of Patent document 1, and Paragraph 0002 of Patent document 2), and they are not intended for a casting die, which is the product that the present invention can be most suitably applied to. Further, neither Patent Documents 1 nor 2 discloses nor suggests carrying out the SP treatment that gives compressive residual stress, after removing a layer of chemical compounds (layer of high carbides) that are produced by a heat treatment (heat hardening treatment).
Patent documents 3-5 disclose technologies to remove an anomalous surface layer (white layer or a compound layer) of a coil or spring that is a steel product and that was treated by a nitride prior to SP treatment, although the technologies do not concern the SP treatment that treats the shaped surface of the die. In these Patent documents, removing the anomalous surface layer is carried out to increase the effect of the SP treatment that gives compressive residual stress.
Patent document 3 discloses a process of manufacturing a coil spring that is treated by the SP treatment after the white layer of a coil product, which white layer is generated by a nitride treatment, is removed by an electrolytic treatment (claim 1, etc.).
The technology to remove the white layer (compound layer) that is disclosed in Patent document 3 is different from the technology of the present invention and it is not directed to a technology to remove the white layer (compound layer) by shots.
Patent document 4 discloses a technology of shot-peening a gear after the white layer is removed by injecting particles of great hardness (shots), which particles have angular shapes, against the bottom of and dedendum of the gear that was treated by a heat treatment that includes a nitride treatment (claim 1, etc.). This patent document does not disclose any improved durability by removing of the layer that is treated by a nitride.
Patent document 5 discloses a technology wherein a diffusion layer of nitrogen is exposed on the gear tooth surface after the compound layer is removed by injecting hard particles that have a hardness of 100 or more in a Vicker's hardness test, against the gear tooth surface made of steel (steel product) that is nitride-treated (claims).
The technology to remove the compound layer that is disclosed in this Patent document is different from the technology of the invention, which comprises two-step treatments, that is, a shot-peening to remove the compound layer and a shot-peening to give compressive residual stress, which are clearly separated from each other.
Further, regarding the treatment of a casting die by nitride, non-patent document 1 contains the following description on pages 103 and 104 in the article that is entitled “4.2.3 The properties that are required of the die for surface treatment.”
“Where the surface to be treated has a clear layer of nitride, the surface that is treated by surface treatment has a stress distribution morphology wherein the surface has a buckling, which causes a lowered residual stress to the surface in the treatment-phase, and then has a maximum stress while the buckling is caused by the compressive stress in the layer treated by nitride. Also, if only a little nitride is formed from a gaseous form, the stress distribution morphology shows that the maximum compressive residual stress is measured at the top of the surface, and gradually declines. This shows a typical embodiment of a surface treatment having a gradient function structure where a casting die that is subjected to heat cycles receives just a little change of residual stress caused by the disintegration of the nitride and shows a favorable mild stress gradient in the surface layer even during a heat cycle between heating and cooling. If a thick layer of nitride is formed, a change of tensile stress is observed when the nitride on the surface disintegrates in the heat cycle. So, a die that was treated by nitride and has a thick layer of nitride on the surface is likely to suffer cracks sooner in operation and they are likely to be regarded as defects on the surface of the die. However, no clear expansion or progress of the cracks is observed in the disintegration process nor is there any clear sign of change in the width of the opening of the cracks if the cycles are repeated.”
As described above, there is no prior-art document that discloses the subject of removing a compound layer by the treatment of the shot.
Also, there is no established method of evaluating or determining that a compound layer is sufficiently removed for steel products, including a gear or a coil, to say nothing of a method for removing a compound layer of a die. Also, the information as to whether the compound layer is sufficiently removed was not in practical use.